Acrylate pressure-sensitive adhesives are well-known in the art. Ulrich (U.S. Reissue No. 24,906) describes alkyl acrylate copolymers which comprise a major portion amount of C.sub.4 to C.sub.14 alkyl esters of acrylic acid monomers and a minor portion of a copolymerizable polar monomer such as acrylic acid. Such adhesives are widely popular due to their availability, their low cost and their ability to provide the requisite fourfold balance of adhesion, cohesion, stretchiness, and elasticity known to be required for effective pressure-sensitive adhesives.
Those skilled in the art have developed acrylate pressure-sensitive adhesives whose properties are tailored in one way or another to meet the demands of specific applications. D. Satas, in Handbook of Pressure Sensitive Adhesive Technology, Van Nostrand Reinhold, New York, 1989, Chapter 15, provides an overview of acrylate pressure sensitive adhesives and their historical development. It is evident that practitioners in the art have employed a wide variety of combinations of acrylic monomers in their search for adhesives having specified properties.
We have identified a need for an acrylate latex pressure sensitive adhesive composition which is completely pH-neutral and which does not contain ionic species of any kind, specifically for the purpose of allowing, within the adhesive matrix, the growth of microorganisms. Typically, emulsion polymerization techniques will employ an anionic emulsifier, or a mixture of anionic and nonionic emulsifiers (Handbook of Pressure Sensitive Adhesive Technology, p. 437), which, in either case, contributes some ionic character to the resulting adhesive.
U.S. Pat. No. 4,565,783, assigned to the assignee of the present case, describes Petrifilm.TM. microorganism assay plates. The liquid inoculum is contained via a foam dam. These plates, although very useful require a degree of care to assemble because of the need for the foam dam. These plates are built up via several layers, increasing the cost of the plate. The adhesive comprises an acrylic adhesive prepared via solution polymerization.
U.S. Pat. No. 4,310,509 (Berglund et al.), assigned to the assignee of the present case, discloses, but does not claim, a solution-polymerized 2-ethylhexyl acrylate/N-vinyl pyrrolidone adhesive that is suspended with an aqueous solution of a broad-spectrum antibiotic prior to use. This patent does not teach an adhesive which is made as an emulsion polymer, after which any additives (such as selective inhibitors, growth promoters, fillers, etc.) are solubilized in the aqueous emulsion and the mixture is coated onto the film backing.
U.S. Pat. No. 3,728,148 discloses a solution-polymerized pressure-sensitive adhesive tape in which the adhesive is free of reactive groups, in order to prevent corrosion of materials on which the tape is applied.
EP 130080, assigned to Johnson and Johnson, describes an aqueous emulsion polymerization of alkyl acrylates, especially butyl acrylate, 2-ethylhexyl acrylate or mixtures thereof, and N-vinylcaprolactam to produce a PSA for surgical products. The preferred emulsifier is anionic, i.e., sodium dihexylsulfosuccinate. A water-soluble persulfate (ionic) initiator is preferred. The monomer N-vinyl pyrrolidone is specifically excluded from use in these PSAs.
JP 58198349 (Sekisui) describes a PSA for therapeutic tapes or sheets which contains no active chemical groups. The adhesive is a mixture of a methacrylic acid ester polymer and an acrylic acid ester polymer, with the proviso that at least one polymer must contain an amide linkage. The reference mentions only "diacetone acrylamide" as the amide monomer. These polymers are made in organic solution, not via emulsion polymerization.
The prior art has thus not provided a totally neutral nonionic pressure sensitive adhesive via an emulsion polymerization process which would be suitable for culture media growth, for application as skin adhesives, for drug delivery, or for the mounting of photographs.
To our knowledge, a latex acrylate adhesive which is pH-neutral, contains no acidic, basic, or reactive moieties and is made exclusively using a nonionic emulsifier has not been previously described. We have discovered such an adhesive and a method for producing it.
U.S. Pat. No. 5,232,838, Nelson et al., assigned to the assignee of the present case, discloses the use of the adhesive of the invention with a microbiological assay device. Since the adhesive is acid/base neutral it allows growth of the species being assayed while exhibiting sufficient adhesive strength to hold agar and other nutrients as well as the liquid inoculum.